Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. In general, the wireless communication system is a multiple access system capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power, etc.). Examples of the multiple access system include a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, etc.
In a typical wireless communication system, only one carrier is considered generally even if a bandwidth is differently set between an uplink and a downlink. The carrier is defined with a center frequency and a bandwidth. Likewise, in 3rd generation partnership project (3GPP) long term evolution (LIE), on the basis of a single carrier, one carrier constitutes each of the uplink and the downlink, and the bandwidth of the uplink is symmetrical to the bandwidth of the downlink in general. However, except for some areas of the world, it is difficult to allocate frequencies of wide bandwidths. Therefore, as a technique for effectively using fragmented small bands, a spectrum aggregation (or carrier aggregation) technique is being developed to obtain the same effect as when a band of a wide bandwidth is logically used by aggregating a plurality of physically non-contiguous bands (i.e., frequency bands) in a frequency domain.
LTE is currently under study in the 3GPP and is one of the latest standards of mobile communication techniques. Wireless access of the LTE is referred to as an evolved-UMTS terrestrial radio access network (E-UTRAN). A 3GPP LTE system uses hybrid automatic repeat request (HARQ) to increase transmission efficiency. Downlink HARQ implies that, when a base station (BS) transmits downlink data, a user equipment (UE) transmits an acknowledgement (ACK)/not-acknowledgement (NACK) signal for the downlink data. Uplink HARQ implies that, when the UE transmits uplink data, the BS transmits an ACK/NACK signal for the uplink data. In the 3GPP LTE, when the UE transmits uplink data on a physical uplink shared channel (PUCCH), the BS sends an ACK/NACK signal on a physical hybrid-ARQ indicator channel (PHICH). The PHICH is a physical channel for carrying HARQ ACK/NACK, and is identified by the UE by using a resource used in PUSCH transmission and a cyclic shift of a demodulation-reference signal (DM-RS) used for demodulation. The section 6.9 of 3GPP TS 36.211 V8.2.0 (2008-03) “Technical specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)” may be incorporated hereby be reference.
The 3GPP LTE considers only a single carrier. Therefore, an uplink carrier used in PUSCH transmission and a downlink carrier used in PHICH transmission are unique. However, there is a problem in that it is difficult to be directly applied to the multiple-carrier system.